The ability to filter specific contaminants from an air stream is particularly useful in air-purifying respirators for individual protection and collective protection air filtration scrubbers. Additionally, reduction in the size of the adsorbent bed is important in reducing physiological burden of the users and improving integration of personal protective equipment including filters in operational environments. By reducing the size of the adsorbent bed, the overall size of the air-purification canister or cartridge can also be reduced. In an air-purifying respirator with mask-mounted filtration element, this reduction in size moves the center of mass of the canister closer to the user's face, reducing the moment of inertia, and increasing user comfort and reducing wearer fatigue. Additionally, a given size of filter may be maintained while increasing the gas capacity of the filter, which provides longer wear times due to time to breakthrough associated with contaminants removed for that size or capacity of filter.
Typically, adsorbents used in canisters are high surface area materials such as activated carbons or zeolites. Carbons readily adsorb many organic species from an air stream; however their limited activity toward high vapor pressure species such as acid gases and ammonia makes removal difficult. To overcome these limitations, the carbon can be impregnated with reactive chemicals specifically targeted to certain agents. There is a balance which must be maintained to improve reactivity of the carbon while maintaining the high surface area required for adsorption. Too high of an impregnation amount will decrease the activity of the carbon. This means that to remove greater amounts of agent, larger amounts of activated carbon are required to maintain sufficient bed residence times for the agent. Current applications overcome this in one of two ways: either a broad spectrum carbon is used, such as Calgon Carbon's Universal Respirator Carbon or URC, in a large, single bed, or multiple layers of activated carbon or impregnated alumina are used, each targeting a specific class of agents.
High surface area materials (carbon, alumina, zeolites) are commonly used for air filtration because they can remove many contaminants. The ability of these materials to remove certain chemicals greatly depends on the chemical composition of the adsorbent, surface area, and pore size. The removal also depends on the chemical and physical properties of the contaminant. These properties determine whether a contaminant is physisorbed or chemisorbed. Physisorbed contaminants are usually only weakly bound and can be released with changes in temperature, gas stream composition, or other conditions. Chemisorbed materials undergo a chemical reaction on the adsorbent surface and are thus converted to a different material. Activated carbons, while containing very high surface areas, are insufficient to adsorb low boiling point compounds such as ammonia, thus some type of impregnant is required to react and trap the contaminant. Impregnation of any support involves a balance of loading a sufficient amount of the reactive impregnant without destroying the high surface area of the support. Some types of metal oxides have shown the capability to adsorb ammonia, but high surface area materials are necessary to adsorb any significant amount.
It would be desirable to provide an improved filter media arrangement for an air purification canister or cartridge that meets current performance requirements and minimizes the overall size of the adsorbent bed. The desired arrangement would reduce the overall size of the canister or cartridge or would allow increased filter gas capacity while maintaining current canister/cartridge size.